Fluid loss reducer for high temperature high pressure water based-mud application

ABSTRACT

The present invention concerns a water-based drilling mud for utilization in the drilling of oil wells comprising an aqueous phase wherein the aqueous phase contains an oil soluble polymer in the form of a gel as fluid loss reducer. The subject invention further reveals a process for preparing an oil soluble polymer fluid loss control agent comprising the steps of dissolving at least one polymer in a hydrocarbon oil to form a clear solution or a gel, adding an emulsifier to the solution or the gel, and keeping the mixture under conditions of agitation until a clear creamy mixture is obtained.

BACKGROUND OF THE INVENTION

[0001] Drilling fluids, commonly referred to as drilling muds, arecomplex mixtures of chemicals. They are required to cool and lubricatethe drill bit, suspend formation cuttings, lift them to the surface, andcontrol formation pressure during the drilling of oil wells.

[0002] For the most part, the liquid bases are aqueous solutions, oilsor emulsions of aqueous and oily materials, to which viscosifiers,dispersants, emulsifiers, weighting agents, fluid loss agents, pHcontrol agents, salts, lubricants, select polymers, corrosioninhibitors, biocides, are usually added to enable the muds to meet theneeds for particular drilling operations.

[0003] Drilling muds are usually classified as either water-based muds(WBM) or oil-based muds (OBM), depending upon the character of thecontinuous phase of the mud, although water-based muds may contain oiland oil-based muds may contain water.

[0004] Oil-based muds generally use hydrocarbon oil as the main liquidcomponent with other materials such as clays or colloidal asphalts addedto provide the desired viscosity together with emulsifiers, polymers andother additives including weighting agents. Water may also be present,but in an amount not usually greater than 50 volume percent of theentire composition. If more than about 5% volume water is present, themud is often referred to as an invert emulsion, i.e., water-in-oilemulsion.

[0005] Water based muds conventionally comprise viscosifiers, fluid losscontrol agents, weighting agents and other additives includinglubricants, emulsifiers, corrosion inhibitors, salts and pH controlagents. The water makes up the continuous phase of the mud and isusually present in any amount of at least 50 volume percent of theentire composition. Oil is also usually present in minor amounts butwill typically not exceed the amount of the water so that the mud willretain its character as a water continuous phase material.

[0006] Potassium-muds are the most widely accepted water mud system fordrilling water-sensitive shales. K⁺ ions attach to clay surfaces andlend stability to shale exposed to drilling fluids by the bit. The ionsalso help hold the cuttings together, minimising dispersion into finerparticles. Potassium chloride (KCl) is the most widely used potassiumsource. Others are potassium acetate, potassium carbonate, potassiumlignite, potassium hydroxide and potassium salt of partial hydrolyzedpolyacrylamide (PHPA). For rheology control, different types of polymersare used, for example XC polymer (Xanthan gum) and PHPA. For fluid-losscontrol, mixtures of starch and polyanionic cellulose are often used.Carboxymethyl starch (CM starch), hydroxy propyl starch (HP starch),carboxymethylcellulose and sodium polyacrylate (SPA) are also used. PHPAis widely used for shale encapsulation.

[0007] Salt-water muds contain varying amounts of dissolved sodiumchloride (NaCl) as a major component. Undissolved salt may also bepresent in saturated salt muds to increase density or to act as abridging agent over permeable zones. Starch and starch derivatives forfluid-loss control and Xanthan gums for hole-cleaning are among the fewhighly effective additives for salt-water muds.

[0008] Sea-water mud is a water based mud designed for offshore drillingwhose make-up water is taken from the ocean. Sea-water has relativelylow salinity, containing about 3 to 4% by weight of NaCl, but has a highhardness because of Mg⁺² and Ca⁺² ions. Hardness is removed from seawater by adding NaOH, which precipitates Mg⁺² as Mg(OH)₂ and by addingNa₂CO₃, which removes Ca⁺² as CaCO₃. Mud additives are the same as thoseused in freshwater muds: bentonite clay, lignosulfonate, lignite,carboxymethylcellulose or polyanionic cellulose and caustic soda. XCpolymer may also be used in place of bentonite.

[0009] Silicate-mud is a type of shale-inhibitive water mud thatcontains sodium or potassium silicate as the inhibitive component. HighpH is a necessary characteristic of silicate muds to control the amountand type of polysilicates that are formed. Mud pH is controlled byaddition of NaOH (or KOH) and the appropriate silicate solution.Silicate anions and colloidal silica gel combine to stabilize thewellbore by sealing microfractures, forming a silica layer on shales andpossibly acting as an osmotic membrane, which can produce in-gauge holesthrough troublesome shale sections that otherwise might require an oilmud.

[0010] Lime-mud is a type of water based mud that is saturated withlime, Ca(OH)₂, and has excess, undissolved lime solids maintained inreserve. Fluid-loss additives include starch, HP-starch,carboxymethylcellulose (CMC) or polyanionic cellulose (PAC).

[0011] Apart from cooling and lubrification of the drilling bit,evacuation of the cuttings to the surface and control of the formationpressure, drillings muds must ensure to minimize invasion into permeablezones.

[0012] The drilling fluid creates a filter cake that imparts lowpermeability to the face of the permeable formation. The ideal filtercake comprises a relatively thin and hard layer as opposed to thickviscous coating. Pressure in the bore hole exceeds the pressure in thepermeable formation and thereby creates the filter cake which furtherresults in liquid from the drilling fluid moving into the permeableformation. This leaves a layer of the filter cake on the face of thehole. Liquid permeating this filter cake and the formation is calledfiltrate. As the thickness of the filter cake increases, the volume offluid loss also increases. The thinner the filter cake, the lower thefluid loss. A thick wall cake will lead not only to high fluid loss, butalso to a reduction in the diameter of the well bore.

[0013] The function of the fluid loss control agents is to delay,prevent or at least limit as far as possible fluid losses that may besustained by the drilling fluids during the drilling operation. However,most of fluid loss control agents used in water based muds such aspolyanionic cellulose, carboxymethylcellulose, starch, etc. give alsorheology to the mud. This is therefore limiting the level of fluid losscontrol agent that can be used. In this context, an additive that wouldonly control the fluid loss properties would allow more flexibility interm of level usage. This can become really critical when severedrilling conditions require an ultra-low permeability barrier betweenthe well bore and the formation.

[0014] Additionally, the compounds added to the mud must withstand thehigh temperature/high pressure (HTHP) in the wells. Materials that aredescribed in the art as HTHP fluid loss control aids have actually apoor stability under these extreme conditions of temperature andpressure.

[0015] For high salinity and high temperature conditions,acrylamido-methyl-propane sulfonate polymers (AMPS polymers) have beendeveloped. However, these polymers are showing limited performance above120° C.

SUMMARY OF THE INVENTION

[0016] The present invention relates to an improved fluid loss reducerfor use in water-based drilling mud. It is a principal object of thepresent invention to provide an improved fluid loss reducer that can beused at high levels and under conditions of high temperature and highpressure in water-based muds.

[0017] This invention is based upon the unexpected discovery that an oilsoluble polymer in a form of a gel can be used as fluid loss additivefor drilling water-based mud compositions. By utilizing such an oilsoluble polymer in drilling water-based muds, greatly improved hightemperature stability and improved shear resistance can be obtained.

[0018] The present invention more specifically discloses a water-baseddrilling mud comprising an aqueous phase wherein the aqueous phasecontains an oil soluble polymer in a form of a gel as fluid lossreducer.

[0019] The water-based mud drilling fluid according to the instantinvention comprises water or salt water, viscosifiers, fluid losscontrol agents, weighting agents and an oily phase, and otherconventional additives selected from the group consisting ofemulsifiers, lubricants, corrosion inhibitors, salts and pH controlagents.

[0020] The present invention more specifically discloses a water-baseddrilling fluid mud comprising:

[0021] (1) from 50 to 90% of the aqueous phase,

[0022] (2) from 0.01 to 0.5% of pH controllers,

[0023] (3) from 0.1 to 5% of viscosifiers,

[0024] (4) from 0.01 to 30% of salts,

[0025] (5) from 0.1 to 3% of emulsifiers,

[0026] (6) from 4 to 60% of weighting agents,

[0027] (7) from 0 to 15% of clays, and

[0028] (8) from 0.1 to 20% of oil soluble polymer in form of a gel asfluid loss reducer, said percentages being based on the weight of themud.

[0029] The subject invention further reveals a process for preparing anoil soluble polymer fluid loss control agent comprising the steps ofdissolving at least one polymer in a hydrocarbon oil to form a clearsolution or a gel, adding an emulsifier to the solution or the gel, andkeeping the mixture under conditions of agitation until a clear creamymixture is obtained.

[0030] The present invention also discloses a method of lubricating adrilling bit during the drilling of a well which comprises circulating awater-based drilling mud in the vicinity of the drilling bit wherein thewater-based drilling mud is comprised of an aqueous phase wherein theaqueous phase contains an oil soluble polymer in the form of a gel as afluid loss reducer.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The water-based drilling fluid mud of the present invention iscomprised of an aqueous phase, wherein the aqueous phase contains an oilsoluble polymer in the form of a gel as a fluid loss reducer. Thewater-based drilling fluid mud will typically be comprised of theaqueous phase, a pH controller, a viscosifier, a salt, an emulsifier, aweighting agent, clay, and an oil soluble polymer in the form of a gelas a fluid loss reducer. This mud composition will normally contain 50%to 90% of the aqueous phase, 0.01% to 0.5% pH controllers, 0.1% to 5%viscosifiers, 0.01% to 30% salts, 0.1% to 3% emulsifiers, 4% to 60%weighting agents, 0% to 15% clays, and 0.1% to 20% of oil solublepolymers in the form of a gel as a fluid loss reducer; based upon theweight of the mud.

[0032] The water-based drilling fluid mud will more typically contain55% to 70% of the aqueous phase, 0.1% to 0.3% pH controllers, 0.4% to 2%viscosifiers, 5% to 15% salts, 0.5% to 2% emulsifiers, 10% to 25%weighting agents, 5% to 10% clays, and 0.5% to 2.5% of oil solublepolymers in the form of a gel as a fluid loss reducer; based upon theweight of the mud.

[0033] The oil soluble polymer is highly efficient in small proportionsas a fluid loss reducer. It may be incorporated in an amount of 0.1 to10%, most preferably in an amount of 0.5 to 2.5% based on the weight ofthe mud.

[0034] The suitable polymers for preparing the emulsion used accordingto the invention are organo-soluble polymers and may be selected fromthe group constituted by the linear polymers, the grafted polymers, thebranched polymers and the cross-linked polymers.

[0035] A wide variety of polymers or copolymers can be utilized in thisinvention. These polymers may be prepared by polymerization using bulk,emulsion, suspension, solution (anionic, cationic, radical, controlledradical), and condensation polymerization technique. Batch,semi-continuous or continuous polymerization processes are suitable forutilization in the synthesis of the polymer.

[0036] The monomers that can be employed are typically selected from thegroup consisting of styrene, substituted styrene, alkyl acrylate,substituted alkyl acrylate, alkyl methacrylate, substituted alkylmethacrylate, acrylonitrile, methacrylonitrile, acrylamide,methacrylamide, N-alkylacrylamide, N-alkylmethacrylamide, isoprene,butadiene, ethylene, vinyl acetate, vinyl ester of versatic acidscontaining from 9 to 19 carbon atoms (C₉ to C₁₉), and combinationsthereof. Functionalized versions of these monomers may also be used.Some representative examples of the selective monomers, which can beused, include styrene, alpha-methylstyrene, para-methylstyrene,para-tertbutylstyrene, vinyltoluene, (M)ethyl(Me)acrylate,2-ethylhexyl(Me)-acrylate, butyl(Me)acrylate, cyclohexyl(Me)acrylate,isobornyl(Me)acrylate, isobutyl(Me)acrylate,p-terbutyl-cyclohexyl(Me)acrylate, butadiene, isoprene, ethylene, veova,vinyl acetate, acid(Me)acrylic, hydroxy-ethyl(Me)acrylate, glycidylmethacrylate, sodium benzenesulfonate and combinations thereof.

[0037] The typical but non-limiting examples of suitable polymers, thatare useful in this invention, are commercially available from a widevariety of sources. For instance, Eliokem sells styrene-acrylate orstyrene-butadiene copolymers under the trade name Pliolite®, such asPliolite® DF01, Pliolite® DF02, Pliolite® DF03, Pliolite® VTAC-H,Pliolite® VT and Pliolite Ultra 200®, substituted styrene-acrylatecopolymers under the trade name Plioway®; Rohm sells acrylate resinsunder the trade name Plex®, Goodyear Chemicals sells styrene-butadienerubber under the trade name Plioflex®; Kraton S.A. sells blockcopolymers styrene-butadiene and their hydrogenated version under thetrade name Kraton®.

[0038] A wide variety of crosslinking agents can be utilized in carryingout the polymerization to produce these polymers. Some representativeexamples of crosslinking agents which can be utilized includedifunctional acrylates, difunctional methacrylates, trifunctionalacrylates, trifunctional methacrylates, allyl maleate anddivinylbenzene.

[0039] Various other components are added to provide the desiredproperties for the drilling mud. Various polymers are used to controlthe mud rheology and therefore to keep the rock cuttings in suspensionas they move up to the borehole to the surface. An alkaline substance,such as caustic soda, provides alkalinity to the system. Weightingagent, such as barite, is also used.

[0040] The water-mud may be a potassium mud, a salt water mud, a seawater mud, or a lime mud.

[0041] The present invention further reveals a process for preparing theoil soluble polymer fluid loss control agent.

[0042] The oil soluble polymer fluid loss control agent may be preparedby dissolving polymers in a hydrocarbon oil to form a clear solution ora gel, adding an emulsifier and keeping the mixture under stirring untila clear creamy mixture is obtained; optionally an emulsion can beprepared by adding water under high shear stirring.

[0043] The oil soluble polymer thus obtained is added to a water-basedmud prepared by conventional methods, either in replacement of theconventional fluid loss reducers, or in addition to said conventionalfluid loss reducers.

[0044] The organic liquids, which may be utilized in the instantinvention, are selected with relation to the polymer solubility. Thehydrocarbon oil is selected from the group comprising of aromatichydrocarbon, chlorinated aliphatic hydrocarbons, aliphatic hydrocarbons,cyclic aliphatic ethers, aliphatic ethers, or organic aliphatic estersand mixture thereof. Preferably hydrocarbon oils are selected from thegroups of synthetic hydrocarbons and organic aliphatic ester, mostpreferably from the group of well fluids (base oil) useful in the rotarydrilling process. The properties of the base oil can vary however, sothat it is usually necessary to perform solubility test to determine theappropriate amount of oil to prepare the emulsion.

[0045] The level of emulsifier added to the polymer solution istypically 1 to 30%, preferably from 3 to 20%, most preferably from 5 to15% based on the weight of the polymer solution/emulsifier mixture. Thepreferred emulsifiers for use in the instant invention include ionic andnon ionic derivatives and mixtures thereof. Specific examples ofpreferred emulsifiers that are useful in the instant invention includealkyl sulfate, alkyl benzene sulfonate, alkyl ethersulfates, sulfonatedoleic acid, alkylphenol ethersulfates, sulfosuccinates, phosphoricester, fatty acid amides, fatty acid amines, fatty alcoholpolyglycolethers, modified fatty alcohol polyglycolethers, alkylpolyglycosides, modified alkyl polyglycosides, alkylphenolpolyglycolethers, fatty acid polyglycolethers, sorbitan fatty acidesters. More preferably, the emulsifier is chosen in the groupcomprising alkyl ethersulfates and fatty acid amides and theirderivatives.

[0046] Optionally, water can be added to the polymer solution/emulsifiermixture. The level of water is typically 0 to 60%, preferably from 10 to50%, most preferably from 20 to 40% based on the weight of the polymeremulsion.

[0047] The level of polymers in the polymer solution is typically about1 to about 40%, preferably from 5 to 35%, most preferably from 15 to 25%based on the weight of the polymer solution. It is important for thepolymer backbones to be soluble in the organic liquid.

[0048] The present invention further reveals the use of an oil solublepolymer in a form of a gel as a fluid loss reducer in a water-based mud.

[0049] The instant invention further reveals a method of lubricating adrill pipe when drilling well, said method comprising circulating awater-based drilling fluid containing an oil soluble polymer in a formof a gel as a fluid loss reducer.

[0050] The oil soluble polymers in a form of a gel, according to theinstant invention, allow a good thermal stability and avoid anyTheological contribution. They may be used at high temperature and highpressure conditions. Moreover, they improve the quality of the filtercake and also improve the water-based muds lubricity performance.

[0051] This invention is illustrated by the following examples that aremerely for the purpose of illustration and are not to be regarded aslimiting the scope of the invention or the manner in which it can bepracticed. Unless specifically indicated otherwise, parts andpercentages are given by weight.

EXAMPLE 1 Preparation of the Oil Emulsion and of the Polymer in a Formof a Gel

[0052] The preparation of an oil emulsion is as follows: First, 100grams of base oil (Ex: Radiagreen®) is mixed with 33.3 grams ofemulsifier (Disponil FES®) and kept under stirring for 5 minutes. Then,36.7 grams of water are slowly incorporated to the oil/emulsifiermixture which is stirred under high shear for 20 minutes.

[0053] If needed, defoamer is added to the preparation. The particlesize of the resulting emulsion is then measured.

[0054] The polymers in a form of a gel are prepared using the samerecipe: 100 grams of base oil is replaced by 100 grams of 80/20 polymergel which is prepared by the dissolution of 20 grams of polymers in 80grams of base oil.

[0055] The polymers used were either linear, grafted, branched orcross-linked.

EXAMPLE 2 Preparation of the Water-Based Muds

[0056] The base muds were prepared by utilizing a conventionallaboratory method. The muds were then placed in aging cells, pressurizedat 6.9×10⁵ Pa (100 psi), and heated for 4 hours at 95° C. (203° F.)and/or 16 hours at 95° C. (203° F.) and/or 16 hours at 130° C. (266°F.). The aging cells were cooled down to room temperature, depressurizedand then the rheology of the mud was measured on a Fann Model 35viscometer at 50° C. (122° F.). Static filtration measurement wasperformed with standard API filtration cell at 6.9×10⁵ Pa (100 psi) and25° C. (77° F.).

[0057] In order to test the emulsion property at high pressure and hightemperature, static filtration measurements at 34.5×10⁵ Pa (500 psi) and95° C. (203° F.) were performed. The results, obtained with thewater-based mud containing the polymeric emulsion, are directly comparedwith the water-based mud (control) and the water-based mud containingthe oil emulsion.

[0058] The different mud formulations used are shown in table I. TABLE IFormulation 1 (water-based mud) Formulation 2 Formulation 3 INGREDIENTS(g) (g) (g) Freshwater 276.5 265.2 265.2 KCl 45 45 45 Caustic Soda 0.30.3 0.3 Soda Ash 0.3 0.3 0.3 Polyanionic 0.75 0.75 — Cellulose 1Polyanionic 3.0 3.0 — Cellulose 2 Xanthan gum 0.75 0.75 1.25 OCMA clay35 35 35 Barite 58.31 58.31 58.31 Emulsion — 31.9 31.9

[0059] Formulation 1 (base-mud) is reference based mud comprising ioniccellulose as fluid loss reducer.

[0060] Formulation 2 is a water-based mud with the same composition asformulation 1, but also containing an oil soluble polymer according tothe instant invention as fluid loss reducer (“on top” formulation). Forthe preparation of formulation 2a, the emulsion does not contain anypolymer. For the preparation of formulation 2b, a linear polymer isused. For the preparation of formulation 2c, a cross-linked polymer isused.

[0061] In formulation 3, polyanionic cellulose is replaced by the oilsoluble polymer according to the instant invention. A cross-linkedpolymer is used.

[0062] The results obtained with the non cross-linked and thecross-linked polymers emulsion over the temperature range of 25° C.-95°C. (77° C.-200° F.) are directly compared with the water-based mud(control).

[0063] As shown in table II, the polymeric emulsion gives significativebetter results in term of static filtration than the referenceformulation without having any impact on mud rheology. TABLE II RheologyFiltration Fann 35 SA viscometer Fann HTHP filter Plastic press 600 300200 200 6 3 viscosity Yield point Room rpm rpm rpm rpm rpm rpm (cps =(lb/100 ft² = t° C. 95° C. Muds formulations (lb/100 ft² = 0.479 Pa)10⁻⁴ Pa · 5) 0.479 Pa) ml ml Formulation 1 78 52 43 31 9 7 26 26 4.013.6 76 51 43 30 8 6 25 26 4.8 14.0 Formulation 2a: Oil emulsion 79 5442 28 7 5 25 29 2.0 10.0 (on top) Formulation 2b: Linear polymer VTACH ®73 48 37 25 6 4 25 23 1.6 8.0 (on top) XPR036 ®¹ 73 53 45 33 13 11 20 332.4 8.8 Pliolite VT ® 78 56 46 33 8 6 22 34 — 8.4 Kraton G ® 86 60 49 348 6 26 34 2.0 8.4 Formulation 2c: Cross-linked Pliolite DF01 ® 75 51 4028 7 5 24 27 1.6 6.4 polymer DF01-NaSS ® 86 59 48 32 7 6 27 32 1.6 6.4(on top) Pliolite DF02 ® 90 62 50 34 8 6 28 34 — 5.6 Pliolite DF03 ® 8961 48 33 7 5 28 33 — 6.0 CPR 7676 ® 88 59 47 32 7 5 29 30 1.6 5.6 CPR7755 ® 88 60 49 33 8 6 28 32 1.6 6.0 Formulation 3: Cross-linkedPliolite DF01 ® 33 18 14 11 4 3 5 3 4.8 — polymer (replacement)

[0064] Filtration value in ml is significantly reduced when polymer suchas VTACH®, U200, XPR036, Pliolite VT®, or Kraton G® was added inre-emulsified form in the standard water-based drilling fluidformulation. This value is further reduced when the above polymer isreplaced by a cross-linked polymer such as Pliolite DF01, DF02, DF03,CPR7676, CPR7755. Replacement in water-based mud of fluid loss reducersby the polymer emulsion leads to equivalent level of filtration versuswater-based mud.

[0065] As it can be seen in Table III, filtration value in ml issignificantly reduced after high temperature aging when cross-linkedpolymer such as Pliolite DF01® was used with an emulsifier package(Disponil FES®/Kleemul®) in the standard water-based drilling fluidformulation. TABLE III Filtration Rheology Fann HTHP Fann 35 SAviscometer filter press 600 300 200 200 6 3 Plastic Yield point Room t°C. rpm rpm rpm rpm rpm rpm viscosity (lb/100 ft² = 95° C. MudsFormulations (lb/100 ft² = 0.479 Pa) (cps) 0.479 Pa) ml After aging 16hours at 95° C. (200° F.) Formulation 1 38 26 20 14 4 3 12 14 14.4Formulation 3c Pliolite DF01 ® 64 45 36 25 7 5 19 26 5.2 After aging 16hours at 130° C. (266° F.) Formulation 1 29 20 16 11 3 2 9 11 20.4Formulation 3c Pliolite DF01 ® 40 27 21 15 4 3 13 14 6.8

[0066] While certain representative embodiments and details have beenshown for the purpose of illustrating the subject invention, it will beapparent to those skilled in this art that various changes andmodifications can be made therein without departing from the scope ofthe subject invention.

What is claimed is:
 1. A water-based drilling mud comprising an aqueousphase wherein the aqueous phase contains an oil soluble polymer in theform of a gel as a fluid loss reducer.
 2. A water-based drilling mud asspecified in claim 1 which is further comprised of water, at least oneviscosifier, at least one fluid loss control agent, at least oneweighting agent, an oily phase, and other conventional additivesselected from the group consisting of emulsifiers, lubricants, corrosioninhibitors, salts and pH control agents.
 3. A water-based drilling mudas specified in claim 2 wherein the water is salt water.
 4. Awater-based drilling mud as specified in claim 1 comprising: (1) from 50to 90% of the aqueous phase, (2) from 0.01 to 0.5% of pH controllers,(3) from 0.1 to 5% of viscosifiers, (4) from 0.01 to 30% of salts, (5)from 0.1 to 3% of emulsifiers, (6) from 4 to 60% of weighting agents,(7) from 0 to 15% of clays, and (8) from 0.1 to 20% of oil solublepolymer in form of a gel as fluid loss reducer, said percentages beingbased on the weight of the mud.
 5. A water-based drilling mud asspecified in claim 1 comprising: (1) from 55 to 70% of the aqueousphase, (2) from 0.1 to 0.3% of pH controllers, (3) from 0.4 to 2% ofviscosifiers, (4) from 0.5 to 15% of salts, (5) from 0.5 to 2% ofemulsifiers, (6) from 10 to 25% of weighting agents, (7) from 5 to 10%of clays, and (8) from 0.5 to 2.5% of oil soluble polymer in form of agel as fluid loss reducer, said percentages being based on the weight ofthe mud.
 6. A water-based drilling mud as specified in claim 1 whereinthe oil soluble polymer is incorporated in an amount of 0.1% to 10%,most preferably in an amount of 0.5% to 2.5%, based on the weight of themud.
 7. A water-based drilling mud as specified in claim 1 wherein thepolymer is an organo-soluble polymer selected from the group consistingof linear polymers, grafted polymers, branched polymers and cross-linkedpolymers.
 8. A water-based drilling mud as specified in claim 1 whereinthe polymer is prepared from monomers selected from the group consistingof styrene, substituted styrene, alkyl acrylate, substituted alkylacrylate, alkyl methacrylate, substituted alkyl methacrylate,acrylonitrile, methacrylonitrile, acrylamide, methacrylamide,N-alkylacrylamide, N-alkylmethacrylamide, isoprene, butadiene, ethylene,vinyl acetate, and vinyl esters of versatic acids containing from 9 to19 carbon atoms.
 9. A water-based drilling mud as specified in claim 1wherein the polymer is prepared from monomers selected from the groupconsisting of styrene, alpha-methylstyrene, para-methylstyrene,para-tertbutylstyrene, vinyltoluene, methyl-acrylate, ethyl-acrylate,methyl-methacrylate, ethyl-methacrylate, 2-ethylhexyl-acrylate,2-ethylhexyl-methacrylate, butyl-acrylate, butyl-methacrylate,cyclohexyl-acrylate, cyclohexyl-methacrylate, isobornyl-acrylate,isobornyl-methacrylate, isobutyl-acrylate, isobutyl-methacrylate,p-tertiary-butyl-cyclohexyl-acrylate,p-tertiary-butyl-cyclohexyl-methacrylate, butadiene, isoprene, ethylene,veova, vinyl acetate, acrylic acid, methacrylic acid,hydroxyethyl-acrylate, hydroxyethyl-methacrylate, glycidyl methacrylate,and sodium benzenesulfonate.
 10. A process for preparing an oil solublepolymer fluid loss control agent comprising the steps of dissolving atleast one polymer in a hydrocarbon oil to form a clear solution or agel, adding an emulsifier to the solution or the gel, and keeping themixture under conditions of agitation until a clear creamy mixture isobtained.
 11. A process as specified in claim 10 wherein the agitationis provided by stirring.
 12. A process as specified in claim 10 whereinwater is added to the mixture under high shear stirring.
 13. A processas specified in claim 10 wherein the hydrocarbon oil is selected fromthe group consisting of aromatic hydrocarbons, chlorinated aliphatichydrocarbons, aliphatic hydrocarbons, cyclic aliphatic ethers, aliphaticethers and organic aliphatic esters.
 14. A process as specified in claim10 wherein the hydrocarbon oil is selected from the group consisting ofsynthetic hydrocarbons and organic aliphatic ester.
 15. A process asspecified in claim 14 wherein the hydrocarbon oil is a mixture of asynthetic hydrocarbon and an organic aliphatic ester.
 16. A process asspecified in claim 10 wherein the emulsifiers is a member selected fromthe group consisting of alkyl sulfates, alkyl benzene sulfonates, alkylethersulfates, sulfonated oleic acid, alkylphenol ethersulfates,sulfosuccinates, phosphoric ester, fatty acid amides, fatty acid amines,fatty alcohol polyglycolethers, modified fatty alcohol polyglycolethers,alkyl polyglycosides, modified alkyl polyglycosides, alkylphenolpolyglycolethers, fatty acid polyglycolethers, and sorbitan fatty acidesters.
 17. A process as specified in claim 10 wherein the emulsifier isselected from the group consisting of alkyl ether sulfates and fattyacid amides.
 18. A method of lubricating a drilling bit during thedrilling of a well which comprises circulating a water-based drillingmud in the vicinity of the drilling bit wherein the water-based drillingmud is comprised of an aqueous phase wherein the aqueous phase containsan oil soluble polymer in the form of a gel as a fluid loss reducer. 19.A method as specified in claim 18 wherein the water-based drilling mudis utilized in conjunction with conventional fluid loss reducers.
 20. Amethod as specified in claim 18 wherein the method is conducted in theabsence of conventional fluid loss reducers.